1. Field of the Invention
The present invention relates to a speed control apparatus for an elevator wherein a cage is driven by supplying an induction motor with alternating current subjected to pulse width modulation.
2. Description of the Prior Art
FIG. 5 is a block diagram in which, by way of example, the arrangement of a prior-art speed control apparatus for an elevator disclosed in the official gazette of Japanese Patent Application Laying-open No. 56-123795 is shown along with a driving system for a cage. Referring to the figure, the alternating current of a three-phase A. C. power source 1 is rectified by a converter 2, and the rectified output is smoothed by a capacitor 3, whereupon alternating current subjected to pulse width modulation by an inverter 4 is applied to an induction motor 5. A speed detector 6 such as tachometer generator, and the sheave 7 of a hoisting machine are directly coupled to the induction motor 5. Further, a cage 9 is coupled to one end of a main rope 8 wound round the sheave 7, and a counterweight 10 to the other end thereof. Besides, the speed command S11 of a speed command generator 11 as a reference for driving the cage at an appropriate speed and the speed detection signal S6 of the speed detector 6 are input to a microcomputer 20, in which for the purpose of the vector control of the induction motor 5, a slip frequency command and a primary current command are calculated and also the instantaneous current command S20 of the induction motor 5 is evaluated. This instantaneous current command is applied to a pulse width modulation circuit (hereinbelow, called "PWM circuit") 13 along with the current detection signal S12 of a current detector 12 which detects the output current of the inverter 4. The PWM circuit 13 calculates the deviation between the instantaneous current command S20 and the current detection signal S12, prepares a pulse width modulation signal for rendering this deviation null, and applies this signal to a base drive circuit 14. The base drive circuit 14 prepares the base signals of transistors constituting the inverter 4 on the basis of the pulse width modulation signal, so as to control the "on" times of the transistors.
The microcomputer 20 is composed of interface circuits 21 and 22 for accepting the speed command S11 and the speed detection signal S6, a microprocessor 23, a ROM 24 and a RAM 25 for storing the data and programs of the microprocessor, and a D/A converter 26 for converting a digital quantity into an analog quantity and delivering the latter as the output.
Since the control circuitry shown in FIG. 5 subjects the induction motor 5 to the slip frequency control, the microcomputer 20 executes the calculation of the following equation: ##EQU1## where I.sub.1 : primary current value,
I.sub.M : secondary excitation current, PA0 L.sub.2 : secondary inductance, PA0 R.sub.2 : secondary resistance, PA0 .omega..sub.s : slip frequency command. PA0 R.sub.20 : secondary resistance at a temperature t.sub.0.
In this regard, the secondary resistance R.sub.2 in Eq. (1) ought to differ depending upon the ambient temperature of the induction motor and the temperature of the rotor thereof and changes in a relationship indicated by the following equation by way of example: ##EQU2## where R.sub.2 : secondary resistance at a temperature t,
Nevertheless, in the prior-art speed control apparatus shown in FIG. 5, a fixed value has been employed as the secondary resistance. This has led to the disadvantage that the overvoltage of the induction motor is incurred in the situation in which the secondary resistance value is large due to the temperature rise of the rotor, whereas the voltage of the induction motor lowers to incur an insufficient torque in the situation in which the secondary resistance value is small due to a low temperature.